The present invention relates to a system and method for a microwave antenna feed for a parabolic reflector. More specifically, the present invention relates to a system and method for a center fed, parabolic reflector feed with enhanced electric (xe2x80x9cExe2x80x9d) and magnetic (xe2x80x9cHxe2x80x9d) plane patterns.
In general, a waveguide is utilized to direct a high-frequency electromagnetic signal rearwardly toward a parabolic reflector for forward reflection. Common reflectors include the two-reflector Cassagrain system in which a horn shaped waveguide directs the signal away forwardly to a sub-reflector from which the signal is directed rearwardly towards the main parabolic reflector for forward reflection. Back-feed, center waveguide systems are also common in which the horn directs the signal rearwardly and directly onto the parabolic reflector for forward reflection.
The signal transmission path from the center of the main reflector is the most important region of the path for obtaining a desired radiation pattern. If there are physical obstructions in this transmission path, an undesirable radiation pattern with side lobes may result. Inherent in the design of the Cassagrain antenna is the problem that the energy transmitted from the main parabolic reflector is blocked by the sub-reflector, and in turn substantial side lobes are generally created. Side lobes also may be formed when there is an uncontrolled destructive combination of two waves or improper control of the E and H plane patterns.
One known back-feed antenna feed system includes a waveguide which has a cap at the distal end for directing the signal rearwardly towards the parabolic reflector from which the waveguide extends. There is generally less obstruction of the transmission path by the cap as compared with a sub-reflector, but waveguide caps generally do not allow for good control and balancing of the E and H plane patterns. Proper control of the E and H plane patterns to avoid large side lobes requires careful placement of the cap with respect to the end of the waveguide and performance problems generally arise because the structurally required proximity of the cap to the waveguide removes the option of strategically locating the cap to control the E and H plane patterns.
Another known back-feed antenna feed system uses a center waveguide bent so as to point toward the parabolic reflector from which it extends. This configuration generally allows for the balancing of E and H plane patterns; however, the obstruction caused by the waveguide geometry generally creates substantial side lobes.
It is also known to split a single waveguide of an antenna feed system into two waveguides that collectively direct an electromagnetic signal rearwardly towards the main reflector. A conventional apparatus with such dual output waveguides is disclosed in U.S. Pat. No. 2,824,305 and includes an input waveguide of rectangular shape that connects to a rectangular head which defines two output waveguides that are substantially parallel to the input waveguide and are of substantially rectangular shape. In such systems, the waveguide is located in the center of the parabolic reflector and the head obstructs the transmission path resulting in a radiation pattern with significant side lobes. In addition, there are two distinctly separate beams of electromagnetic energy directed towards the parabolic reflector; the effects of a point source illuminating the parabolic reflector with a single beam of electromagnetic energy cannot be replicated by such a waveguide.
Accordingly, it is an object of the present invention to obviate many of the above problems in the known systems and to provide a novel system and method for a center fed reflector feed with enhanced E and H plane patterns.
It is another object of the present invention to provide a novel center fed reflector feed apparatus and method with reduced obstruction of the transmitted signal by the waveguide.
It is yet another object of the present invention to provide a novel center fed reflector feed apparatus and method that matches the impedance between a single input waveguide and dual output waveguides.
It is still another object of the present invention to provide a novel center fed reflector feed apparatus and method that matches the impedance between the waveguide and free space.
It is a further object of the present invention to provide a novel dual waveguide center fed reflector feed apparatus and method that generates an improved radiation pattern.